This invention relates to a radio receiver having an automatic gain control (AGC) circuit controlling the gain of at least an intermediate frequency (IF) stage and having a logarithmic generating circuit for providing an output signal that is a logarithm of the AGC signal that is applied directly to the IF stage.
It is well known to employ an AGC circuit in a radio receiver to automatically adjust the gain in one or more of the amplifier stages so as to keep the radio output signal at about the same level while the amplitude of the incoming radio signal may vary over many orders of magnitude. Normally the AGC circuit does not begin to attenuate the IF radio signal until the incoming radio signal exceeds an amplitude substantially greater than the usual background noise. This AGC threshold action is designed to occur only for radio signals that are strong enough to provide good quality radio reception, i.e. clear noise free sound. Beyond that "minimum strength" radio signal, the AGC control signal (voltage or current) is adjusted so that the IF stage is biased to operate in its optimum mode of highest gain without unwanted overdriving and saturation.
If the radio signal strength increases even further, then the AGC signal increases to hold the output of the IF stage essentially constant. The AGC signal as a function of radio signal strength is quite nonlinear.
A second AGC circuit is often provided to control the gain of the RF stages, usually including the mixer stage. This second AGC circuit usually has an even higher action threshold, and a greater delay.
Thus not only are there many sources of nonlinearity in these radio gain controlling circuits, but no one gain controlling signal contains all the information from which the strength of the incoming radio signal can be deduced.
Now it was recognized long ago that for providing a visual indication of radio signal strength, it would be desirable to progressively compress the indicator signal more and more as the radio signal strength rises higher and higher. Attempts to effect such compression have led to signal strength meter circuits that are very roughly logarithmic. Such circuits are normally located at the input to or at an intermediate position within the AGC circuitry.
There is a need for a logarithmic signal strength indicator circuit that covers about four decades of medium to high radio-signal strength, e.g. antenna voltages from 10 to 100,000 .mu.v (micro volts), especially for use in mobile cellular narrow-band FM radio receivers and in stereo AM receivers. During operation a cellular radio receiver periodically samples the signal from the nearby cellular radio transmitters and compares their signal strengths to determine which is strongest (at least 6 db greater than the others). The receiver automatically communicates with the station having the strongest signal. To make such a signal strength comparison of two signals near 10 .mu.v with the same accuracy that a comparison of two signals at 100,000.mu. volts are made is difficult and is believed never to have been done before. Such a system would also provide a superior radio-signal strength indicator for monitoring a wide range of medium to high strength signals or for stopping tune scanners.
It is therefore a primary object of this invention to provide a circuit that provides an output that is an accurate logarithm of the input signal over a wide range of signal amplitudes.
It is a further object of this invention to provide a logarithm (LN) circuit that provides two (or more) outputs that are accurate logarithms of two (or more) input signals respectively, and which circuit sums the two (or more) output signals to provide the LN of the product of the two (or more) input signals.
It is further an object of this invention to provide such a circuit in a radio wherein the two circuit input signals are the outputs from an IF AGC circuit and RF AGC circuit respectively so that the sum of the LN circuit outputs is essentially a LN function of the incoming radio signal.